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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
from this point, down to the middle of the ileum, they diminish considerably in size. In the lower part of the ileum they almost entirely disappear; hence the comparative thinness of this portion of the intestine, as compared with the duodenum and jejunum. The circular folds retard the passage of the food along the intestines, and afford an increased surface for absorption.


FIG. 1063– Vertical section of a human aggregated lymphatic nodule, injected through its lymphatic canals. a. Villi with their chyle passages. b. Intestinal glands. c. Muscularis mucosæ. d. Cupola or apex of solitary nodule. e. Mesial zone of nodule. f. Base of nodule. g. Points of exit of the lacteals from the villi, and entrance into the true mucous membrane. h. Retiform arrangement of the lymphatics in the mesial zone. i. Course of the latter at the base of the nodule. k. Confluence of the lymphatics opening into the vessels of the submucous tissue. l. Follicular tissue of the latter. (See enlarged image)

  The intestinal villi (villi intestinales) are highly vascular processes, projecting from the mucous membrane of the small intestine throughout its whole extent, and giving to its surface a velvety appearance. They are largest and most numerous in the duodenum and jejunum, and become fewer and smaller in the ileum.


FIG. 1064– Transverse section through the equatorial plane of three aggregated lymphatic nodules from the rabbit. (See enlarged image)

  Structure of the villi (Figs. 1059, 1060).—The essential parts of a villus are: the lacteal vessel, the bloodvessels, the epithelium, the basement membrane, and the muscular tissue of the mucosa, all being supported and held together by retiform lymphoid tissue.
  The lacteals are in some cases double, and in some animals multiple, but usually there is a single vessel. Situated in the axis of the villus, each commences by dilated cecal extremities near to, but not quite at, the summit of the villus. The walls are composed of a single layer of endothelial cells.
  The muscular fibers are derived from the muscularis mucosæ, and are arranged in longitudinal bundless around the lacteal vessel, extending from the base to the summit of the villus, and giving off, laterally, individual muscle cells, which are enclosed by the reticulum, and by it are attached to the basement-membrane and to the lacteal.
  The bloodvessels (Fig. 1061) form a plexus under the basement membrane, and are enclosed in the reticular tissue.
  These structures are surrounded by the basement membrane, which is made up of a stratum of endothelial cells, and upon this is placed a layer of columnar epithelium, the characteristics of which have been described. The retiform tissue forms a net-work (Fig. 1060) in the meshes of which a number of leucocytes are found.
  The intestinal glands (glandulæ intestinales [Lieberkühni]; crypts of Lieberkühn) (Fig. 1062) are in considerable numbers over every part of the mucous membrane of the small intestine. They consist of minute tubular depressions of the mucous membrane, arranged perpendicularly to the surface, upon which they open by small circular apertures. They may be seen with the aid

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